Surface Science Studies Research Articles

Surface structures of ZrO2 films on Rh(111): From two layers to bulk termination

We have studied zirconia films on a Rh(111) substrate with thicknesses in the range of 2–10 monolayers (ML) using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Zirconia was deposited using a UHV-compatible sputter source, resulting in layer-by-layer growth and good uniformity of the films. For thicknesses of 2–4 ML, a layer-dependent influence of the substrate on the structure of the thin films is observed. Above this thickness, films show a (2 × 1) or a distorted (2 × 2) surface structure with respect to cubic ZrO2(111); these structures correspond to tetragonal and monoclinic zirconia, respectively. The tetragonal phase occurs for annealing temperatures of up to 730 °C; transformation to the thermodynamically stable monoclinic phase occurs after annealing at 850 °C or above. High-temperature annealing also breaks up the films and exposes the Rh(111) substrate. We argue that the tetragonal films are stabilized by the interface to the substrate and possibly oxygen deficiency, while the monoclinic films are only weakly defective and show band bending at defects and grain boundaries. This observation is in agreement with positive charge being responsible for the grain-boundary blocking effect in zirconia-based solid electrolytes. Our work introduces the tetragonal and monoclinic 5 ML-thick ZrO2 films on Rh(111) as a well-suited model system for surface-science studies on ZrO2, as they do not exhibit the charging problems of thicker films or the bulk material and show better homogeneity and stability than the previously-studied ZrO2/Pt(111) system.

In-Situ Observations of Anodic Alumina Nanopore Growth and Self-Organisation

Traditional surface science studies have been quite successful in determining several surface structures for alumina films, in particular those grown on NiAl, where the bulk structure consists of a combination of several building blocks but the surface Al atoms are found to occupy sites with either tetrahedral or pyramidal coordination [1]. However, the naturally forming ‘native oxide’ layers are found to be largely amorphous. The thickness of this layer can be increased electrochemically, i.e. by the industrial process of anodization, and to some extent the degree of crystallinity found depends on the applied voltage [2]. This type of oxide layer is called an anodic barrier layer and forms in neutral type electrolytes. In acidic electrolytes a porous oxide film is found, in which case the oxide has many nanometer sized pores and forms the base of most industrial aluminum coloring processes. Furthermore, under certain conditions these pores are found to be self-ordering [3]. They form hexagonal arrays and the pore diameter can be tuned by changes in potential, making excellent nanoscale templates. We have investigated the formation and self-ordering behavior of porous type anodic alumina films in situ using Grazing Transmission Small-Angle X-Ray Scattering (GTSAXS) [5]. We observe the that the in-plane arrangement of the nanopores is independent of substrate crystallographic orientation whereas the oxide growth rate is not. The self-ordering behavior is studied in a variety of electrolytes and at several potentials in-order to explore the dynamics of self-ordering both below and above the breakdown potential (UB) at which optimal ordering is achieved [4]. It is also possible to follow the chemical etching of the oxide (pore widening) and subsequent metal deposition within the nanopores. The experimental approach presented can be applied to the study of a large variety of electrochemically produced materials such as magnetic nanowires, novel solar cell designs and catalysts. [1] G. Prevot, et al. Phys. Rev. B, 85, 205450 (2012) [2] J.W. Diggle, et al. Chem. Revs., 69(3), 365 (1969) [3] H. Masuda, K. Fukuda, Science. 268, 1466–1468 (1995) [4] Chu S.Z, et al. A. J. Electrochem Soc., 153, B384 (2006) [5] Vinogradov, N.Am et al. ACS Applied Nano Materials, Article ASAP, (2018) Figure 1: GTSAXS pattern during a 2nd anodoization of PAA. The oscillations correspond to the height of the pores. Figure 1

Flying couplers above spinning resonators generate irreversible refraction.

Creating optical components that allow light to propagate in only one direction-that is, that allow non-reciprocal propagation or 'isolation' of light-is important for a range of applications. Non-reciprocal propagation of sound can be achieved simply by using mechanical components that spin1,2. Spinning also affects de Broglie waves 3 , so a similar idea could be applied in optics. However, the extreme rotation rates that would be required, owing to light travelling much faster than sound, lead to unwanted wobbling. This wobbling makes it difficult to maintain the separation between the spinning devices and the couplers to within tolerance ranges of several nanometres, which is essential for critical coupling4,5. Consequently, previous applications of optical6-17 and optomechanical10,17-20 isolation have used alternative methods. In hard-drive technology, the magnetic read heads of a hard-disk drive fly aerodynamically above the rapidly rotating disk with nanometre precision, separated by a thin film of air with near-zerodrag that acts as a lubrication layer 21 . Inspired by this, here we report the fabrication of photonic couplers (tapered fibresthat couple light into the resonators) that similarly fly above spherical resonators with a separation of only a few nanometres. The resonators spin fast enough to split their counter-circulating optical modes, making the fibre coupler transparent from one side while simultaneously opaque from the other-that is, generating irreversible transmission. Our setup provides 99.6 per cent isolation of light in standard telecommunication fibres, of the type used for fibre-based quantum interconnects 22 . Unlike flat geometries, such as between a magnetic head and spinning disk, the saddle-like, convex geometry of the fibre and sphere in our setup makes it relatively easy to bring the two closer together, which could enable surface-science studies at nanometre-scale separations.

Sub-Angstrom Gold Nanoparticle/Liposome Interfaces Controlled by Halides.

A hallmark of nanoscience is size-dependent and distance-dependent physical properties. Although most previous studies focused on optical properties, which are often tuned at nanometer scale, we herein report on the interaction between halide-capped gold nanoparticles (AuNPs) and phosphocholine (PC) liposomes at the sub-Angstrom level. Halide-capped AuNPs are adsorbed by PC liposomes attributable to van der Waals force. Iodide-capped AuNPs interact much more weakly with the liposomes compared with bromide- and chloride-capped AuNPs, as indicated by a liposome leakage assay and differential scanning calorimetry. This is explained by the slightly larger size of iodide separating the AuNP core more from the liposome surface. Cryo-transmission electron microscopy indicates that the liposomes remain intact when mixed with these halide-capped AuNPs of 13 or 70 nm in diameter. Other even larger ligands, including small thiol compounds, DNA oligonucleotides, proteins, and polymers, fully blocked the interaction, whereas AuNPs dispersed in noninteracting ions, including fluoride, phosphate, perchlorate, nitrate, sulfate, and bicarbonate, are still adsorbed strongly by 1,2-dioleoyl- sn-glycero-3-phosphocholine liposomes. Taken together, halides can be used to control interparticle distances at an extremely small scale with remarkable effects on materials properties, allowing surface probing, biosensor development, and fundamental surface science studies.

Mechanism-based design of precursors for focused electron beam-induced deposition

Focused electron beam-induced deposition (FEBID) is capable of producing metal-containing nanostructures with lateral resolution on the sub-nanometer scale. Practical application of this nanofabrication technique has been hindered by ligand-derived contamination from precursors developed for thermal deposition methods. Mechanistic insight into FEBID through surface science studies and gas-phase electron–molecule interactions has begun to enable the design of custom FEBID precursors. These studies have shown that precursors designed to decompose under electron irradiation can produce high-purity FEBID deposits. Herein, we highlight the progress in FEBID precursor development with several examples that incorporate this mechanism-based design approach.

Switch in Relative Stability between cis and trans 2-Butene on Pt(111) as a Function of Experimental Conditions: A Density Functional Theory Study

The adsorption of cis and trans 2-butenes on Pt(111) has been studied as a function of hydrogen coverage θH by means of calculations based on density functional theory (DFT) with the inclusion of dispersion forces. All hydrogen coverages have been considered, from 0 to 1.00 monolayer (ML). For each case, the di-σ and π adsorption geometries of the olefins have been compared at a surface coverage of θC4H8 = 0.11 ML. Calculations of the Gibbs free energies of these systems have identified the most stable 2-butene isomer (cis or trans) as a function of coverage, temperature, and pressure. In particular, focus was placed on two sets of conditions, namely, one with a pressure of 10–7 Torr, a temperature of 80 K, and a gas ratio (PH2/Pbutene) of 25, similar to the conditions used in surface-science studies, and a second with a pressure of 1 bar, a temperature range of 300–400 K, and a gas ratio (PH2/Pbutene) of 10, similar to catalytic hydrogenation conditions. With all selected functionals (PW91, PBE-TS, and optPBE), di-σ bonding was found to be the most stable for both isomers of 2-butene and for all hydrogen coverages except for θH = 1.00 ML. At low pressures, 2-butene is physisorbed at low temperatures (≤125 K with PBE-TS and ≤90 K with optPBE); however, when the temperature increases, coadsorption of the butene with 6 H atoms becomes the most stable configuration of the system (θH = 0.67 ML), and finally, 2-butene desorbs around 380 K, as estimated with PBE-TS (or around 325 K with optPBE). Interestingly, a switch in stability was observed with hydrogen coverage, from the adsorbed trans isomer being the more stable for θH < 0.44 ML to the adsorbed cis isomer becoming the more stable at higher hydrogen coverages, in agreement with the cis–trans isomerization behavior previously reported for this system. At high pressures, the behavior is similar, but with transitions occurring at higher temperatures. 2-Butene is physisorbed until the temperature reaches 250 K, and desorbs above 500 K. At hydrogenation reaction temperatures (between 300 and 500 K), a hydrogen coverage of roughly half a monolayer was calculated (0.66 and 0.44 for 300 and 500 K, respectively). Our results confirm that dispersion effects must be included to properly describe the 2-butene and hydrogen coadsorption on Pt(111), as PW91 predicts that 2-butene is never adsorbed on the platinum surface. On the other hand, DFT calculations including dispersion forces such as PBE-TS or optPBE afford a good understanding of catalytic systems under both ultra-high-vacuum conditions and catalytic hydrogenation conditions. For this system, the PBE-TS results are in good agreement with experiments: they correctly reproduce the coverage in hydrogen and the configuration of the 2-butene adsorbate (cis–trans isomer).

Infrared Reflection–Absorption Spectroscopy and Density Functional Theory Investigations of Ultrathin ZnO Films Formed on Ag(111)

Reactivity at metal-oxide interfaces is of fundamental importance in heterogeneous catalysis. Herein, we report a thorough surface-science study on the growth and chemical activity of ultrathin ZnO...

The most stable adsorption geometries of two chiral modifiers on Pt(111)

The molecular description of chirality transfer on chirally modified metal surfaces is unclear due to the complexity of the systems and the weak energetic biases favoring specific enantioselective pathways. In order to move to an enhanced level of molecular understanding it is critical to define the chemisorption geometries of the modifiers responsible for enantiodifferentiation. Model surface science studies of chiral molecules on single crystals provide an indirect method to probe catalytic chirality transfer molecular mechanisms. Apart from the great difference between ultrahigh vacuum (UHV) and reaction conditions, the extent to which such studies can inform the interpretation of data obtained under reaction conditions depends on the degree to which the model systems are correctly defined. The same holds true for any comparison of UHV surface spectroscopy data to spectroscopic measurements made at the catalyst-solution interface under either in-situ or operando conditions.We present detailed reflection absorption infrared spectroscopy (RAIRS) data on (R)-1-(1-naphthyl)ethylamine, (R)-NEA, and its simple derivative, (R)-1-(8-methyl-1-naphthyl)ethylamine, on Pt(111). Various conflicting proposals for the structure of NEA and analogous modifiers on Pt catalyst particles and extended Pt surfaces are found in the literature. Here, we find that below high sub-monolayer coverages on Pt(111), (R)-NEA adopts a chemisorption geometry where the entire naphthyl group is π-bonded to the surface and the amine group forms a dative bond to the surface. The same general adsorption geometry is found for the methyl-substituted derivative. The study provides reference spectra for these chiral modifiers and confirms the molecular structures described in our previous studies of diastereomeric complexes formed by the (R)-NEA/Pt(111) system.

Electron interactions with the heteronuclear carbonyl precursor H2FeRu3(CO)13 and comparison with HFeCo3(CO)12: from fundamental gas phase and surface science studies to focused electron beam induced deposition.

In the current contribution we present a comprehensive study on the heteronuclear carbonyl complex H2FeRu3(CO)13 covering its low energy electron induced fragmentation in the gas phase through dissociative electron attachment (DEA) and dissociative ionization (DI), its decomposition when adsorbed on a surface under controlled ultrahigh vacuum (UHV) conditions and exposed to irradiation with 500 eV electrons, and its performance in focused electron beam induced deposition (FEBID) at room temperature under HV conditions. The performance of this precursor in FEBID is poor, resulting in maximum metal content of 26 atom % under optimized conditions. Furthermore, the Ru/Fe ratio in the FEBID deposit (≈3.5) is higher than the 3:1 ratio predicted. This is somewhat surprising as in recent FEBID studies on a structurally similar bimetallic precursor, HFeCo3(CO)12, metal contents of about 80 atom % is achievable on a routine basis and the deposits are found to maintain the initial Co/Fe ratio. Low temperature (≈213 K) surface science studies on thin films of H2FeRu3(CO)13 demonstrate that electron stimulated decomposition leads to significant CO desorption (average of 8–9 CO groups per molecule) to form partially decarbonylated intermediates. However, once formed these intermediates are largely unaffected by either further electron irradiation or annealing to room temperature, with a predicted metal content similar to what is observed in FEBID. Furthermore, gas phase experiments indicate formation of Fe(CO)4 from H2FeRu3(CO)13 upon low energy electron interaction. This fragment could desorb at room temperature under high vacuum conditions, which may explain the slight increase in the Ru/Fe ratio of deposits in FEBID. With the combination of gas phase experiments, surface science studies and actual FEBID experiments, we can offer new insights into the low energy electron induced decomposition of this precursor and how this is reflected in the relatively poor performance of H2FeRu3(CO)13 as compared to the structurally similar HFeCo3(CO)12.

Dehydration Pathway for the Dissociation of Gas-Phase Formic Acid on Pt(111) Surface Observed via Ambient-Pressure XPS

While model studies of surface science under ultrahigh vacuum (UHV) have made significant contributions to understanding electrochemistry, many issues related to electrochemical phenomena still remain unanswered due to the extreme environmental differences between UHV and liquid conditions. Electrochemical formic acid (HCOOH) oxidation is one such example. While the dehydration step in the indirect oxidation pathway (HCOOH → H2O + COad → 2H+ + 2e– + CO2) is observed in the electrochemical oxidation of formic acid on Pt(111) surface, the surface science studies conducted in UHV condition reported the complete HCOOH dissociation to H2 and CO2 on Pt(111) surface with no adsorbed CO at room temperature. A dehydration mechanism may also exist in gas-phase HCOOH dissociation in some conditions different from UHV, but it has not been demonstrated with a surface science method due to pressure limitations. Using ambient pressure X-ray photoelectron spectroscopy (AP-XPS), we observed the dehydration mechanism of ga...

Pt/Cu single-atom alloys as coke-resistant catalysts for efficient C–H activation

The recent availability of shale gas has led to a renewed interest in C-H bond activation as the first step towards the synthesis of fuels and fine chemicals. Heterogeneous catalysts based on Ni and Pt can perform this chemistry, but deactivate easily due to coke formation. Cu-based catalysts are not practical due to high C-H activation barriers, but their weaker binding to adsorbates offers resilience to coking. Using Pt/Cu single-atom alloys (SAAs), we examine C-H activation in a number of systems including methyl groups, methane and butane using a combination of simulations, surface science and catalysis studies. We find that Pt/Cu SAAs activate C-H bonds more efficiently than Cu, are stable for days under realistic operating conditions, and avoid the problem of coking typically encountered with Pt. Pt/Cu SAAs therefore offer a new approach to coke-resistant C-H activation chemistry, with the added economic benefit that the precious metal is diluted at the atomic limit.

Photoelectron Spectroscopic Study of Methanol Adsorbed Rutile TiO2(110) Surface

Methanol/TiO2(110) is a model system in the surface science study of photocatalysis where methanol is taken as a hole capture. However, the highest occupied molecular orbital of adsorbed methanol lies below the valence band maximum of TiO2, preventing the hole transfer. To study the level alignment of this system, electronic structure of methanol covered TiO2(110) surface has been measured by ultraviolet photoelectron spectroscopy and the molecular orbitals of adsorbed methanol have been clearly identified. The results indicate the weak interaction between methanol and TiO2 substrate. The static electronic structure also suggests the mismatch of the energy levels. These static experiments have been performed without band gap excitation which is the prerequisite of a photocatalytic process. Future study of the transient electronic structure using time-resolved UPS has also been discussed.

Surface science studies of the diffusion of adsorbed and intercalated lithium

Due to its small size and high ionization affinity, lithium exhibits excellent diffusion properties making it a proper candidate for intercalation ion batteries. We briefly review the experimental surface science studies on the diffusion of Li on solid surfaces, including our own work, and the first nm-scale studies of Li intercalation. Particular emphasis has been placed on the presentation of the surface science approach including the thorough preparation of the substrate surface in UHV, the control of the surface structure and chemical composition before and during the experiments, and the critical quantitative consideration of external factors such as temperature, electric or magnetic field, or the presence of reactive gases. Using Li as an exemplary case, typical methods and experimental procedures employed in surface diffusion studies are presented, that may eventually lead to a better understanding and further improvements of the transport properties of adsorbed and intercalated Li.

O2 Activation on Ceria Catalysts-The Importance of Substrate Crystallographic Orientation.

An atomic-level understanding of dioxygen activation on metal oxides remains one of the major challenges in heterogeneous catalysis. By performing a thorough surface-science study of all three low-index single-crystal surfaces of ceria, probably the most important redox catalysts, we provide a direct spectroscopic characterization of reactive dioxygen species at defect sites on the reduced ceria (110) and (100) surfaces. Surprisingly, neither of these superoxo and peroxo species was found on ceria (111), the thermodynamically most stable surface of this oxide. Applying density functional theory, we could relate these apparently inconsistent findings to a sub-surface diffusion of O vacancies on (111) substrates, but not on the less-closely packed surfaces. These observations resolve a long standing debate concerning the location of O vacancies on ceria surfaces and the activation of O2 on ceria powders.

Planar model system of the Phillips (Cr/SiO2) catalyst based on a well-defined thin silicate film

The Phillips catalyst (Cr/SiO2) is successfully used in the large-scale production of polyethylene and has attracted a great interest in catalytic community over the last sixty years. However, the atomic structure of the active site(s) and the reaction mechanism remain controversial, in particular due to the structural complexity and surface heterogeneity of the amorphous silica. In this work, we used a well-defined, atomically flat silicate bilayer film grown on Ru(0001) as a support offering the opportunity to investigate mechanistic aspects at the atomic scale. To fabricate a planar Cr/SiO2 model system suitable for surface science studies, chromium was deposited using physical vapor deposition onto the hydroxylated silica film surface. Structural characterization and adsorption studies were performed by infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD). Hydroxyls groups seem to serve as anchoring cites to Cr ad-atoms. As monitored by IRAS, hydroxyls consumption correlated with the appearance of the new band at ∼1007 cm−1 typical for CrO vibrations. In addition, CO titration experiments suggested also the presence of “naked” Cr, which transforms into mono- and di-oxo chromyl species and their aggregation upon oxidation treatments. TPD experiments of ethylene adsorption at low temperatures under UHV conditions showed the formation of butane as one of the main products. The resultant surfaces are thermally stable, at least, up to 400 K which allows to investigate ethylene polymerization further under more realistic conditions.

Photocatalytic C[sbnd]C bond cleavage in ethylene glycol on TiO2: A molecular level picture and the effect of metal nanoparticles

Polyol conversion to value-added products is of great interest for the bio-diesel industry. Photocatalytic oxidation processes may offer a green approach for polyol conversion; however the lack of comprehensive mechanistic understanding from an interdisciplinary perspective limits or even misleads the design of highly selective and efficient photocatalysts for such process. Here we have studied the photocatalytic polyol conversion on pristine TiO2 and metal (Au, Pd, and Pt) nanoparticles (NPs) decorated TiO2 using ethylene glycol (EG) as the model compound. We have developed a mechanistic picture at molecular level by coupling in-situ surface science study on rutile (110) surface with in-situ vibrational-mass spectrometry study on TiO2 nanopowders. The CC bond cleavage was found to be the only pathway in EG photo-conversion under deaerated conditions, leading to the formation of formaldehyde and hydrogen. We rationalized that the desorption of the surface adsorbed H (Hads) to be the rate determining step (RDS), making pristine TiO2 a poor photocatalyst that only catalyze the EG conversion at very low surface coverages. The addition of metal NPs on TiO2 surface promotes the desorption of Hads significantly, thus leading to an enhanced CC bond cleavage performance at higher surface coverages that is more applicable.

Surface Reaction Barriometry: Methane Dissociation on Flat and Stepped Transition-Metal Surfaces.

Accurately simulating heterogeneously catalyzed reactions requires reliable barriers for molecules reacting at defects on metal surfaces, such as steps. However, first-principles methods capable of computing these barriers to chemical accuracy have yet to be demonstrated. We show that state-resolved molecular beam experiments combined with ab initio molecular dynamics using specific reaction parameter density functional theory (SRP-DFT) can determine the molecule-metal surface interaction with the required reliability. Crucially, SRP-DFT exhibits transferability: the functional devised for methane reacting on a flat (111) face of Pt (and Ni) also describes its reaction on stepped Pt(211) with chemical accuracy. Our approach can help bridge the materials gap between fundamental surface science studies on regular surfaces and heterogeneous catalysis in which defected surfaces are important.

Catalytic Semihydrogenation of Acetylene on the (100) Surface of the o-Al13Co4 Quasicrystalline Approximant: Density Functional Theory Study

Density functional theory calculations are used to investigate the catalytic properties of the o-Al13Co4(100) surface toward the semihydrogenation of acetylene. The dissociation of the H2 molecule is the key process in this case, since the surface termination does not contain protruding Co atoms, according to a combination of surface science studies and ab initio calculations. However, compared to the pure Al(100) and Al(111) surfaces, H2 dissociation on o-Al13Co4(100) proceeds much easier, due to the presence of surface Co atoms slightly below the mean position of the termination plane. A possible mechanism for the hydrogenation is presented, highlighting the selectivity of the reaction on this Al-rich surface.

Ring‐Opening Reaction of Furfural and Tetrahydrofurfuryl Alcohol on Hydrogen‐Predosed Iridium(1 1 1) and Cobalt/Iridium(1 1 1) Surfaces

AbstractThe ring‐opening of furfural is crucial for its conversion into value‐added chemicals such as 1,5‐pentanediol (1,5‐PeD), an important polymer precursor. Ir‐based catalysts have shown activity in ring‐opening reactions, and the use of an oxophilic metal additive is reported to be beneficial. To better understand the effect of an oxophilic metal on Ir for the opening of the furan ring, a combination of DFT calculations and experimental surface science studies were performed to investigate the ring‐opening of furfural and tetrahydrofurfuryl alcohol. The experimental results suggest that Co/Ir(1 1 1) has a higher ring‐opening activity for an unsaturated furan ring, however, the difference in activities between Ir and Co/Ir(1 1 1) is insignificant for a saturated furan ring. The DFT results reveal that Co/Ir(1 1 1) has a stronger interaction with an unsaturated furan ring than Ir(1 1 1); both surfaces show a weaker interaction with the saturated furan ring, which causes the lower ring‐opening activity than that of furfural.

Epitaxial NixPd1-x (111) Alloy Substrates with Continuously Tunable Lattice Constants for 2D Materials Growth.

Epitaxial strain can be a powerful parameter for directing the growth of thin films. Unfortunately, conventional materials only offer discrete choices for setting the lattice strain. In this work, it is demonstrated that epitaxial growth of transition metal alloy solid solutions can provide thermally stable, high-quality growth substrates with continuously tunable lattice constants. Molecular beam epitaxy was used to grow NixPd1-x(111) alloy films with lattice constants between 3.61 and 3.89 Å on the hexagonal (0001) basal planes of α-Al2O3 and Cr2O3 (grown as epitaxial films on α-Al2O3 (0001)). The Cr2O3 acted as an adhesion layer, which not only improved the high-temperature stability of the films but also produced single-domain films with NixPd1-x [112̅] parallel to Cr2O3 [112̅0], in contrast to growth on α-Al2O3 that yielded twinned films. Surface characterization by electron diffraction and scanning tunneling microscopy (STM) as well as bulk X-ray diffraction analysis indicated that the films are suitable as inexpensive and stable substrates for thin-film growth and for surface science studies. To demonstrate this suitability, bilayer SiO2, a two-dimensional van der Waals material, was grown on a NixPd1-x(111) film tuned to closely match the film's lattice constant, with STM and electron diffraction results revealing a highly ordered, single-phase crystalline state.